This invention relates to geophysical prospecting and exploration, and more particularly relates to methods and means for defining velocities of acoustic energy of sedimentary rocks and determining the causal effects thereof attributable to lithology and post depositional processes.
It is well known in the prior art that any investment by the oil industry in exploration or development within a complex geological environment should be accurately risked to assess the chances of success or failure. Since the majority of the oil industry's prospecting in such environments results in failure and waste, it has not heretofore been known how to systematically and reproducibly improve hydrocarbon prospecting methodology.
Probably a significant proportion of this limitation of the prior art is attributable to depth conversion deficiencies. As is well known in the art and described by Al Chalabi in "Velocity Determination from Seismic Reflection Data" published in Applied Science, vol. 1, pp. 1-68, in 1979, depth conversion is the process of converting time surfaces, defined by seismic data and preferably well control data, to depth surfaces. Velocities used in this conversion are normally accurate only at well control points. Between well control points velocities are derived from the seismic stacking process in which a correction for normal move-out allows an estimate of local velocities to be made. The accuracy of such velocities may locally be of the order of 80% to 100%, and variations with a particular hydrocarbon prospect are likely to be no less than 5%. Such accuracy may be inadequate for drilling purposes, i.e., to locate economically developable hydrocarbons, if either a drilling discovery is subsequently proved to be different from that prognosed in size or distribution, or if a drilling failure, because of defective depth conversion, is actually adjacent a subsequent discovery.
This limitation of the prior art may also be attributable to a failure to correctly assess local presence of sources or seal or reservoir. Seismic stratigraphic studies piece together various items of physical evidence, including velocities of seismic sequences. Subtleties of variation in evidence may be all that is available to determine the difference between, for example, a low energy shale with source potential and a medium energy coarser clastic with no source potential. Thus, effective velocity interpretations should be a part of a seismic stratigraphic analysis which examines such subtleties of evidence. See volume 2 of Tucker's text Applied Geophysics for Exploration for a general discussion of this technology.
While the attempts to advance the prior art have had only limited success, the underlying lithologies from which a plethora of geophysical data is accumulated at great expense during and pursuant to prospecting and the like, physically exists. Indeed, conventional exploration practice can offer an adequate understanding of these factors and their concomitant effects upon velocity distribution, which are apparent in simple basins. In complex multilayered basins, however, especially where stress directions have generally and locally changed with time, the conventional practice often fails to adequately understand and describe these factors.
Even if some of these factors are clear when viewed in isolation, if such were possible, it should be apparent that they are apt to be obscured when associated with each other. Accordingly, what would be useful is a seismic stratigraphic process capable of rigorously isolating, understanding and defining each of these factors, and thereby preventing compounding of the underlying physical reasons which, in the aggregate, cause the lateral and vertical variations in a basin's overburden velocity field.
It is, of course, well within the skill and experience of those skilled in the art to identify and resolve individual problems in some geophysical environments. Data quality is regularly enhanced by on-going improvements in drilling accuracy, advanced processing of improved two-dimensional seismic data, acquisition of closely-spaced seismic lines processed heuristically as three-dimensional data, and generation of depth seismic displays created before and after stack. Nevertheless, even though seismic resolution and signal-to-noise ratios improve at the hands of those skilled in the art, a significant proportion of wells drilled are still failures because of locally miscalculated velocity distribution. Clearly, these improvements are limited to time data, and do not reach velocity distribution. Indeed, as should be apparent to those skilled in the art, even in productive basins which have an increase with time of well control and high quality seismic data, a proportional increase in drilling accuracy has not commonly been realized. Unfortunately, there are still geological surprises under these presumably well defined circumstances.
The rigorous attention to detail for defining sequence related velocities from well and seismic data discussed by Carter in the paper entitled "Depth Conversion Using Normalized Interval Velocities" published in the January 1989 issue of Geophysics TLE, and other recognized experts in the art, affords practitioners the opportunity to reduce the risks of drilling failures. However, since the seismic method intrinsically cannot produce 100% accurate velocity maps, it clearly cannot be the whole answer.
For instance, additional information has been sought related to compaction behavior. More particularly, Gardner et al. in the papers "Formation, Velocity and Density - the Diagnostic Basis for Stratigraphic Traps" published in Geophysics, vol. 39, no. 6, pp. 770-780, and "Elastic Wave Velocities in Heterogeneous and Porous Media" published in Geophysics, vol. 21, no. 1, pp. 41-70, has encouraged practitioners in the exploration field to attempt to remove the effects of burial from sequence velocity maps by normalization techniques. See Carter, "Depth Conversion Using Normalized Interval Velocities" published in Geophysics TLE, January 1989.
Rigorous attention to detail in analyzing well control of sequences pursuant to distinguishing between normal and abnormal behavior may significantly reduce drilling risks, but is nonetheless dependent upon the logic of the method applied. As is known in the art, a commonly applied method to determine compaction behavior is to plot sequence interval velocity at its midpoint depth, per well, and then to statistically interrogate the resulting scattergram to seek trends and exceptions thereto. This scattergram approach, however, inherently precludes insight into local subtle anomalies. For instance, for sequences on the order of four hundred feet thick and sampled every six inches, this conventional method reduces the data to merely one point. In addition, regression analysis performed on individual well midpoints may yield erroneous information about a sequence's compaction, which is contrary to a major trend exhibited by the individual wells.
Bulat and Stoker in their paper entitled "Uplift Determination from Interval Velocity Studies" published in Petroleum Geology of Northwest Europe, pp. 293-305 in 1987, demonstrate some of the limitations of the midpoint depth method. Their investigation into velocity distribution, using techniques well known in the prior art, examined inversion in the UK Gas Basin, and therefor necessarily grouped myriad factors together instead of isolating individual variables. Accordingly, the results were unsatisfactory because of contaminated compaction factors and the like. That is, it may be that the maps obtained were contaminated by 4! variations, i.e., twenty four potential variations, instead of merely being related to basin inversion as possibly intended by Bulat and Stoker. Stated differently, as many as twenty four factors should have been individually considered as contributing to the lithology of the basin, not just basin inversion. In their concluding remarks, the authors observe that their problems were due to the confounding effects of other geologic factors, presumably operating collectively in an obscure and unknown manner.
There have been various attempts to overcome the accuracy limitations inherent in seismic velocities. In U.S. Pat. No. 4,692,910, Sondergeld et al. disclose a method for determining lithological characteristics of an underground formation whereby compressional velocity and shear velocity are plotted against such seismic parameters as bulk velocity, porosity, fluid saturation. Velocity boundaries are determined for at least one formation material type. Thus, from the position of the data points relative to the velocity boundaries, lithological characteristics may be ascertained. But it should be clear to those skilled in the art that the Sondergeld method fails to allow for past depositional processes with their consequences upon lithology.
In U.S. Pat. No. 4,571,710, Neidell teaches a method for identifying zones of anomalous low subsurface velocity based upon seismic reflection arrival time data. Assuming no local lateral velocity variation, the method attempts to relate porosity and the possible presence of hydrocarbons to these zones. Other attempts have been made to improve the determination of local rock composition from seismic data. For instance, Gassaway et al., in U.S. Pat. No. 4,373,197, disclose an exploration system for enhancing the likelihood of predicting lithology of earth formations associated with deposits of ore, marker rock and economic minerals. the system purportedly provides for the accurate mapping of crustal earth formations via refractive seismic waves to identify structure, elastic parameters and lithology of strata whereby mineral deposits and the like may be located. For plots of compressional and shear waves, changes in sediment structure are indicated. There is no provision for defining velocity-depth relationships nor all of the post depositional causes of lithological change observed. Similarly, Ostrander, in U.S. Pat. No. 4,562,558, teaches a method for determining local lithology of gas-bearing strata using high-intensity amplitude events in seismic records. Changes in p-wave reflection coefficient as a function of angle of incidence indicate the lithology of the reflecting horizon and copying strata. However, while this analysis may be useful for identifying anomalies over short distances, it fails to identify possible causes of such anomalies.
Thus, heretofore unknown in the prior art is a method for predicting velocity variation changes based upon knowledge of subsurface geology. On the contrary, the emphasis in the prior art has been to attempt to predict geology based upon knowledge of velocity or magnetic fields.
Furthermore, there is also unknown in the prior art a methodology which focuses on the probable underlying cause or causes for modification of velocities in subsurface sedimentary rock formations.
Accordingly, these limitations and disadvantages of the prior art are overcome with the present invention, and improved means and techniques are provided which are especially useful for geophysical prospecting and exploration by systematically evaluating and analyzing the post depositional causes for velocity variations.